How to remove terrain ground texture?

I have seen in some maps, like chess, that the ground was completly removed so you could see sky 360 degree around you. How to do this? My new map is about a 3D space shooter and it kinda sucks to have visible ground...

Make a new 128x128 completely alpha-ed .blp image and import it to over-ride said tile. Viola! No ground!

Thanks! but i dont know how to do that....can you give me any links to softwares or guides please? =) also what is alpha-ed?

There is this one.

Alpha is transparency.